It means there is a hollow in the point. Looks like a little volcano maybe? It helps to spread the energy of the mass of the bullet, flattening it out upon impact. Of course this is for increased stopping power. Imagine a needle going through you. Now imagine a pennie going through you on the flat side, not the long. Bullet is wider because of the flattening taking more mass out of target. (You, to keep the same example) i.e. stopping power.